Hybrid drive powertrains utilize electric motors to substitute, augment, and recover energy from other sources of driving energy, such as from an internal combustion engine. An electric motor may be utilized by attaching the motor to a drivetrain through a transmission device, such as a transmission gear set. Use of an electric motor in this manner enables the control systems of a vehicle to utilize various control schemes aimed at improving fuel efficiency of the vehicle. However, those having ordinary skill in the art will appreciate that any mechanical system attached to a drivetrain by a set of gears increases overall losses to friction and requires more energy to accelerate due to an increase in rotational inertia. An electric motor and gears associated with attachment to a drivetrain spin with a spinning drivetrain regardless of whether the electric motor is in use or not. These inefficiencies, or parasitic losses, caused by spinning the unused electric motor reduce the overall fuel effectiveness of employing a hybrid drive powertrain.
A control system monitors various inputs from the vehicle and the operator and provides operational control of the powertrain, including controlling transmission operating state and gear shifting, controlling torque-generative devices, and regulating the power interchange among the energy storage device and the torque machines to manage outputs of the transmission, including torque and rotational speed. Known multi-mode hybrid transmission can use differential gearing, torque transfer clutches, and the torque machines to transfer power to an output member that can be connected to a driveline when the powertrain is applied to a vehicle. Torque transfer clutches generally include wet clutches to change speed ratios in the transmission, and hydraulic systems are in turn used to control the wet clutches. Hydraulic systems are complex, costly, and require many components including the particularly bulky and difficult to manufacture valve bodies.